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Abstract:

A locking system for mechanical joining of floorboards constructed from a
body, a rear balancing layer and an upper surface layer. A strip, which
is integrally formed with the body of the floorboard and which projects
from a joint plane and under an adjoining board has a locking element
which engages a locking groove in the rear side of the adjoining board.
The joint edge provided with the strip is modified with respect to the
balancing layer, for example by means of machining of the balancing layer
under the strip, in order to prevent deflection of the strip caused by
changes in relative humidity. Also, a floorboard provided with such a
locking system, as well as a method for making floorboards with such a
locking system.

Claims:

1. A locking system for mechanical joining of floorboards of the type
having a body, opposite first and second joint edge portions and a
balancing layer on the rear side of the body, adjoining floorboards in a
mechanically joined position having their first and second joint edge
portions joined at a vertical joint plane, said locking system
comprising: a) for vertical joining of the first joint edge portion of
the first floorboard and the second joint edge portion of an adjoining
second floorboard mechanically cooperating connectors in the form of a
tongue groove formed in the first joint edge portion and a tongue formed
in the second joint edge portion, and b) for horizontal joining of the
first joint edge of a first floorboard and the second joint edge portion
of the adjoining second floorboard mechanically cooperating connectors
comprising a locking groove formed in the underside of said second
floorboard and extending parallel to and at a distance from the vertical
joint plane at said second joint edge portion and having a downward
opening, and a strip integrally formed with the body of said first
floorboard, which strip at said first joint edge portion projects from
said vertical joint plane and at a distance from the vertical joint plane
has a locking element, which projects towards a horizontal plane
containing the upper side of said first floorboard and which has at least
one operative locking surface for coaction with said locking groove, the
strip forming a horizontal extension of the first joint edge portion
below the tongue groove, wherein the first joint edge portion, within an
area defined by the bottom of the tongue groove and the locking surface
of the locking element, is modified with respect to the balancing layer.

2. A locking system according to claim 1, wherein the balancing layer,
within said area of the first joint edge portion, is lacking or is wholly
or partially removed.

3. A locking system according to claim 1, wherein the balancing layer
with-in said area of the first joint edge portion is modified with
respect to its properties, compared with the properties of the balancing
layer within the remaining parts of the floorboard.

4. A locking system according to claim 1, wherein essentially the entire
area is modified with respect to the balancing layer.

5. A locking system according to claim 1, wherein said area is modified
with respect to the balancing layer across only a part of its horizontal
extent.

6. A locking system according to claim 5, wherein said area is modified
with respect to the balancing layer across more than half of its
horizontal extent.

7. A locking system according to claim 1, wherein the first joint edge
portion is modified with respect to the balancing layer also in a second
area under the locking element.

8. A locking system according to claim 1, wherein the first joint edge
portion exhibits a non-modified balancing layer in a second area under
the locking element.

9. A locking system according to claim 1, wherein said modification
refers to an alteration of the thickness of the balancing layer.

10. A locking system according to claim 1, wherein said area has no
balancing layer at all across at least part of its horizontal extent.

11. A locking system according to claim 1, wherein said area, across its
whole horizontal extent or a part thereof, exhibits a balancing layer
with reduced thickness.

12. A locking system according to claim 1, wherein the first joint edge
portion is modified within said area with respect to the material
composition of the balancing layer.

13. A locking system according to claim 1, wherein the first joint edge
portion within said area is modified with respect to the material
properties of the balancing layer.

14. A locking system according to claim 1, wherein the locking system is
designed in such a way that the tongue is anglable into the tongue groove
and the locking element is insertable into the locking groove by a mutual
angular movement of the first and the second floorboard while maintaining
contact between joint edge surface portions of the floorboards close to
the boundary line between the vertical joint plane and the upper side of
the floorboards.

15. A locking system according to claim 1, wherein the floorboards on the
upper side of the body have a surface layer which coacts with the
balancing layer.

16. A locking system according to claim 1, wherein the tongue groove has
a tongue groove depth which is less than 0.4 times the thickness of the
floorboard, and wherein the strip has a width which is less than 1.3
times the thickness of the floorboard.

17. A locking system according to claim 1, wherein the locking surface of
the locking element has a vertical extent which is at least 0.1 times the
thickness of the floorboard.

18. A locking system according to claim 1, wherein the locking surface of
the locking element is inclined relative to the horizontal plane at an
angle exceeding 45.degree..

19. A locking system according to claim 1, wherein the tongue groove
exhibits an outer part with a vertical height and an inner, narrower part
with a vertical height whose average value across the horizontal extent
of the inner part is less than 0.8 times the vertical height of the outer
part.

20. A locking system according to claim 1, wherein the locking surface of
the locking element has a vertical extent which is less than 0.2 times
the thickness of the floorboard.

21. A locking system according to claim 1, wherein the strip, across at
least half of the part of the strip which in the horizontal direction is
located between the locking surface and the joint edge of the other
floorboard, exhibits a strip thickness which is less than 0.25 times the
thickness of the floorboard.

22. A floorboard provided with a locking system according to claim 1.

23. A floorboard according to claim 22, which is mechanically joinable to
adjoining floorboards along all four sides by a strip-lock system.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a continuation of U.S. application Ser.
No. 12/959,971, filed on Dec. 3, 2010, which is a continuation of U.S.
application Ser. No. 11/822,698, filed on Jul. 9, 2007, which is a
continuation of U.S. application Ser. No. 09/954,064, filed on Sep. 18,
2001, which is a continuation of Application No. PCT/SE00/00785, filed on
Apr. 26, 2000 claims the benefit of Swedish Application No. 9901574-5,
filed on Apr. 30, 1999. The entire contents of each of U.S. application
Ser. No. 12/959,971, U.S. application Ser. No. 11/822,698, U.S.
application Ser. No. 09/954,064, Application No. PCT/SE00/00785 and
Swedish Application No. 0303273-7 are hereby incorporated herein by
reference.

TECHNICAL FIELD

[0002] The invention generally relates to the field of mechanical locking
of floorboards. The invention relates to an improved locking system for
mechanical locking of floorboards, a floorboard provided with such an
improved locking system, as well as a method for making such floorboards.
The invention generally relates to an improvement to a locking system of
the type described and shown in WO 94/26999.

[0003] More specifically, the invention relates to a locking system for
mechanical joining of floorboards of the type having a body, opposite
first and second joint edge portions and a balancing layer on a rear side
of the body, adjoining floorboards in a mechanically joined position
having their first and second joint edge portions joined at a vertical
joint plane, said locking system comprising

[0004] a) for vertical joining of the first joint edge portion of the
first floorboard and the second joint edge portion of the adjoining
floorboard mechanically cooperating means in the form of a tongue groove
formed in the first joint edge portion and a tongue formed in the second
joint edge portion,

[0005] b) for horizontal joining of the first joint edge portion of the
first floorboard and the second joint edge portion of an adjoining
floorboard mechanically cooperating means, which comprise

[0006] a locking groove which is formed in the underside of said second
floorboard and which extends parallel to and at a distance from the
vertical joint plane at said second joint edge portion and which has a
downward opening, and

[0007] a strip made in one piece with the body of said first floorboard,
which strip at said first joint edge portion projects from said vertical
joint plane and at a distance from the joint plane has a locking element,
which projects towards a plane containing the upper side of said first
floorboard and which has at least one operative locking surface for
coaction with said locking groove, and

[0008] said strip forming a horizontal extension of the first joint edge
portion below the tongue groove.

FIELD OF APPLICATION OF THE INVENTION

[0009] The present invention is particularly suitable for mechanical
joining of thin floating floorboards made up of an upper surface layer,
an intermediate fibreboard body and a lower balancing layer, such as
laminate flooring and veneer flooring with a fibreboard body. Therefore,
the following description of the state of the art, problems associated
with known systems, and the objects and features of the invention will,
as a non-restricting example, focus on this field of application and, in
particular, on rectangular floorboards with dimensions of about 1.2 m*0.2
m and a thickness of about 7-10 mm, intended to be mechanically joined at
the long side as well as the short side.

BACKGROUND OF THE INVENTION

[0010] Thin laminate flooring and wood veneer flooring are usually
composed of a body consisting of a 6-9 mm fibreboard, a 0.2-0.8-mm-thick
upper surface layer and a 0.1-0.6 mm lower balancing layer. The surface
layer provides appearance and durability to the floorboards. The body
provides stability, and the balancing layer keeps the board level when
the relative humidity (RH) varies during the year. The RH can vary
between 15% and 90%. Conventional floorboards of this type are usually
joined by means of glued tongue-and-groove joints at the long and short
sides. When laying the floor, the boards are brought together
horizontally, whereby a projecting tongue along the joint edge of a first
board is introduced into the tongue groove along the joint edge of a
second board. The same method is used on both the long and the short
side. The tongue and the tongue groove are designed for such horizontal
joining only and with special regard to how the glue pockets and gluing
surfaces should be designed to enable the tongue to be efficiently glued
within the tongue groove. The tongue-and-groove joint presents coacting
upper and lower contact surfaces that position the boards vertically in
order to ensure a level surface of the finished floor.

[0011] In addition to such conventional floors which are connected by
means of glued tongue-and-groove joints, floorboards have recently been
developed which are instead mechanically joined and which do not require
the use of glue. This type of a mechanical joint system is hereinafter
referred to as a "strip-lock system" since the most characteristic
component of this system is a projecting strip which supports a locking
element.

[0012] WO 94/26999 (Applicant Valinge Aluminum AB) discloses a strip-lock
system for joining building panels, particularly floorboards. This
locking system allows the boards to be locked mechanically at right
angles to as well parallel to the principal plane of the boards at the
long side as well as at the short side. Methods for making such
floorboards are disclosed in WO 98/24994 and WO 98/24995. The basic
principles of the design and the installation of the floorboards, as well
as the methods for making the same, as described in the three
above-mentioned documents are usable for the present invention as well,
and, therefore, these documents are hereby incorporated by reference.

[0013] In order to facilitate the understanding and description of the
present invention, as well as the comprehension of the problems
underlying the invention, a brief description of the basic design and
function of the floorboards according to the above-mentioned WO 9426999
will be given below with reference to FIGS. 1-3 in the accompanying
drawings. Where applicable, the following description of the prior art
also applies to the embodiments of the present invention described below.

[0014] FIGS. 3a and 3b are thus a bottom view and a top view respectively
of a known floorboard 1. The board 1 is rectangular with a top side 2, an
underside 3, two opposite long sides 4a, 4b forming joint edges, and two
opposite short sides 5a, 5b forming joint edges.

[0015] Without the use of glue, both the long sides 4a, 4b and the short
sides 5a, 5b can be joined mechanically in a direction D2 in FIG. 1c. For
this purpose, the board 1 has a flat strip 6, mounted at the factory,
projecting horizontally from its long side 4a, which strip extends
throughout the length of the long side 4a and which is made of flexible,
resilient sheet aluminum. The strip 6 can be fixed mechanically according
to the embodiment shown, or by means of glue, or in some other way. Other
strip materials can be used, such as sheets of other metals, as well as
aluminum or plastic sections. Alternatively, the strip 6 may be made in
one piece with the board 1, for example by suitable working of the body
of the board 1. Thus, the present invention is usable for floorboards in
which the strip is integrally formed with the board. At any rate, the
strip 6 should always be integrated with the board 1, i.e. it should
never be mounted on the board 1 in connection with the laying of the
floor. The strip 6 can have a width of about 30 mm and a thickness of
about 0.5 mm. A similar, but shorter strip 6' is provided along one short
side 5a of the board 1. The edge side of the strip 4 facing away from the
joint edge 4a is formed with a locking element 8 extending throughout the
length of the strip 6. The locking element 8 has an operative locking
surface 10 facing the joint edge 4a and having a height of e.g. 0.5 mm.
When the floor is being laid, this locking surface 10 coacts with a
locking groove 14 formed in the underside 3 of the opposite long side 4b
of an adjoining board 1'. The short side strip 6' is provided with a
corresponding locking element 8', and the opposite short side 5b has a
corresponding locking groove 14'.

[0016] Moreover, for mechanical joining of both the long sides and the
short sides also in the vertical direction (direction D1 in FIG. 1c), the
board 1 is formed with a laterally open recess 16 along one long side 4a
and one short side 5a. At the bottom, the recess is defined by the
respective strips 6, 6'. At the opposite edges 4b and 5b, there is an
upper recess 18 defining a locking tongue 20 coacting with the recess 16
(see FIG. 2a).

[0017] FIGS. 1a-1c show how two long sides 4a, 4b of two such boards 1, 1'
on an underlay U can be joined together by means of downward angling.
FIGS. 2a-2c show how the short sides 5a, 5b of the boards 1, 1' can be
joined together by snap action. The long sides 4a, 4b can be joined
together by means of both methods, while the short sides 5a, 5b--when the
first row has been laid--are normally joined together subsequent to
joining together the long sides 4a, 4b and by means of snap action only.

[0018] When a new board 1' and a previously installed board 1 are to be
joined together along their long sides 4a, 4b as shown in FIGS. 1a-1c,
the long side 4b of the new board 1' is pressed against the long side 4a
of the previous board 1 as shown in FIG. 1a, so that the locking tongue
20 is introduced into the recess 16. The board 1' is then angled
downwards towards the subfloor 12 as shown in FIG. 1b. In this
connection, the locking tongue 20 enters the recess 16 completely, while
the locking element 8 of the strip 6 enters the locking groove 14. During
this downward angling the upper part 9 of the locking member 8 can be
operative and provide guiding of the new board 1' towards the previously
installed board 1. In the joined position as shown in FIG. 1c, the boards
1, 1' are locked in both the direction D1 and the direction D2 along
their long sides 4a, 4b, but can be mutually displaced in the
longitudinal direction of the joint along the long sides 4a, 4b.

[0019] FIGS. 2a-2c show how the short sides 5a and 5b of the boards 1, 1'
can be mechanically joined in the direction D1 as well as the direction
D2 by moving the new board 1' towards the previously installed board 1
essentially horizontally. Specifically, this can be carried out
subsequent to joining the long side of the new board 1' to a previously
installed board in an adjoining row by means of the method according to
FIGS. 1a-1c. In the first step in FIG. 2a, beveled surfaces adjacent to
the recess 16 and the locking tongue 20 respectively co-operate such that
the strip 6' is forced to move downwards as a direct result of the
bringing together of the short sides 5a, 5b. During the final urging
together of the short sides, the strip 6' snaps up when the locking
element 8' enters the locking groove 14'.

[0020] By repeating the steps shown in FIGS. 1a-c and 2a-c, the whole
floor can be laid without the use of glue and along all joint edges.
Known floorboards of the above-mentioned type are thus mechanically
joined usually by first angling them downwards on the long side, and when
the long side has been secured, snapping the short sides together by
means of horizontal displacement along the long side. The boards 1, 1'
can be taken up in the reverse order of laying without causing any damage
to the joint, and be laid again. These laying principles are also
applicable to the present invention.

[0021] For optimal function, subsequent to being joined together, the
boards should be capable of assuming a position along their long sides in
which a small play can exist between the locking surface 10 and the
locking groove 14. Reference is made to WO 9426999 for a more detailed
description of this play.

[0022] In addition to what is known from the above-mentioned patent
specifications, a licensee of Valinge Aluminum AB, Norske Skog Flooring
AS (NSF), introduced a laminated floor with mechanical joining according
to WO 9426999 in January 1996 in connection with the Domotex trade fair
in Hannover, Germany. This laminated floor, which is marketed under the
brand name Alloc®, is 7.2 mm thick and has a 0.6-mm aluminum strip 6
which is mechanically attached on the tongue side. The operative locking
surface 10 of the locking element 8 has an inclination (hereinafter
termed locking angle) of 80° to the plane of the board. The
vertical connection is designed as a modified tongue-and-groove joint,
the term "modified" referring to the possibility of bringing the tongue
and tongue groove together by way of angling.

[0023] WO 97/47834 (Applicant Unilin) describes a strip-lock system which
has a fibreboard strip and is essentially based on the above known
principles. In the corresponding product, "Uniclic", which this applicant
began marketing in the latter part of 1997, one seeks to achieve biasing
of the boards. This results in high friction and makes it difficult to
angle the boards together and to displace them. The document shows
several embodiments of the locking system. The "Uniclic" product, shown
in section in FIG. 4b, consists of a floorboard having a thickness of 8.1
mm with a strip having a width of 5.8 mm, comprising an upper part made
of fibreboard and a lower part composed of the balancing layer of the
floorboard. The strip has a locking element 0.7 mm in height with a
locking angle of 45°. The vertical connection consists of a tongue
and a tongue groove having a tongue groove depth of 4.2 mm.

[0024] Other known locking systems for mechanical joining of board
materials are described in, for example, GB-A-2,256,023 showing
unilateral mechanical joining for providing an expansion joint in a wood
panel for outdoor use, and in U.S. Pat. No. 4,426,820 showing a
mechanical locking system for plastic sports floors, which floor however
does not permit displacement and locking of the short sides by snap
action. In both these known locking systems the boards are uniform and do
not have a separate surface layer and balancing layer.

[0025] In the autumn of 1998, NSF introduced a 7.2-mm laminated floor with
a strip-lock system which comprises a fibreboard strip and is
manufactured in accordance with WO 9426999. This laminated floor, which
is shown in cross-section in FIG. 4a, is marketed under the brand name of
"Fiboloc®". In this case, too, the strip comprises an upper part of
fibreboard and a lower part composed of a balancing layer. The strip is
10.0 mm wide, the height of the locking element is 1.3 mm and the locking
angle is 60°. The depth of the tongue groove is 3.0 mm.

[0026] In January 1999, Kronotex introduced a 7.8 mm thick laminated floor
with a strip lock under the brand name "Isilock". This system is shown in
cross-section in FIG. 4c. In this floor, too, the strip is composed of
fibreboard and a balancing layer. The strip is 4.0 mm and the tongue
groove depth is 3.6 mm. "Isilock" has two locking ridges having a height
of 0.3 mm and with locking angles of 40°. The locking system has
low tensile strength, and the floor is difficult to install.

SUMMARY OF THE INVENTION

[0027] Although the floor according to WO 94/26999 and the floor sold
under the brand name Fiboloc® exhibit major advantages in comparison
with traditional, glued floors, further improvements are desirable mainly
by way of cost savings which can be achieved by reducing the width of the
fibreboard strip from the present 10 mm. A narrower strip has the
advantage of producing less material waste in connection with the forming
of the strip. However, this has not been possible since narrower strips
of the Uniclic and Isilock type have produced inferior test results. The
reason for this is that narrow strips require a small angle of the
locking surface of the locking element in relation to the horizontal
plane (termed locking angle) in order to enable the boards to be joined
together by means of angling, since the locking groove follows an arc
having its centre in the upper joint edge of the board. The height of the
locking element must also be reduced since narrow strips are not as
flexible, rendering snap action more difficult.

[0028] To sum up, narrow strips have the advantage that material waste is
reduced, but the drawbacks that the locking angle must be small to permit
angling and that the locking element must be low to permit joining by
snap action.

[0029] In repeated laying trials and tests with the same batch of
floorboards we have discovered that strip locks, which have a joint
geometry similar to that in FIGS. 4b and 4c, and are composed of a narrow
fibreboard strip with a balancing layer on its rear side and with a
locking element having a small locking surface with a low locking angle,
exhibit a considerable number of properties which are not constant and
which can vary substantially in the same floorboard at different points
in time when laying trials have been performed. These problems and the
reason behind the problems are not known.

[0030] Moreover, at present there are no known products or methods which
afford adequate solutions to these problems which are related to

[0031] (i) mechanical strength of the joint of floorboards with a
mechanical locking system of the strip lock type;

[0032] (ii) handling and laying of such floorboards;

[0033] (iii) properties of a finished, joined floor made of such
floorboards.

(i) Strength

[0034] At a certain point in time, the joint system of the floorboards has
adequate strength. In repeated testing at a different point in time, the
strength of the same floorboard may be considerably lower, and the
locking element slides out of the locking groove relatively easily when
the floor is subjected to tensile stress transversely of the joint.

(ii) Handling/Laying

[0035] At certain times during the year the boards can be joined together,
while at other times it is very difficult to join the same floorboard.
There is a considerable risk of damage to the joint system in the form of
cracking.

(iii) Properties of the Joined Floor

[0036] The quality of the joint in the form of the gap between the upper
joint edges of the floorboards when subjected to stress varies for the
same floorboard at different times during the year.

[0037] It is known that floorboards expand and shrink during the year when
the relative humidity RH changes. Expansion and shrinking are 10 times
greater transversely of the direction of the fibres than in the direction
of the fibres. Since both joint edges of the joint system change by the
same amount essentially simultaneously, the expansion and the shrinking
cannot explain the undesirable effects which severely limit the chances
of providing a strip-lock system at a low cost which at the same time is
of high quality with respect to strength, laying properties, and the
quality of the joint. According to generally known theories, wide strips
should expand more and cause greater problems. Our tests indicate that
the reverse is the case.

[0038] In sum, there is a great need for a strip-lock system which to a
greater extent than the prior art takes into account the above-mentioned
requirements, problems and wishes. It is an object of the invention to
fulfill this need.

[0039] These and other objects of the invention are achieved by a locking
system, a floorboard, and a manufacturing method exhibiting the
properties stated in the appended independent claims, preferred
embodiments being stated in the dependent claims.

[0040] The invention is based on a first insight according to which the
problems identified are essentially connected to the fact that the strip
which is integrated with the body bends upwards and downwards when the RH
changes. Moreover, the invention is based on the insight that, as a
result of its design, the strip is unbalanced and acts as a bimetal.
When, in a decrease of the RH, the rear balancing layer of the strip
shrinks more than the fibreboard part of the strip, the entire strip will
bend backwards, i.e. downwards. Such strip-bending can be as great as
about 0.2 mm. A locking element having a small operative locking surface,
e.g. 0.5 mm, and a low locking angle, e.g. 45 degrees, will then cause a
play in the upper part of the horizontal locking system, which means that
the locking element of the strip easily slides out of the locking groove.
If the strip is straight or slopes upward it will be extremely difficult
to lay the floor if the locking system is adapted to a curved strip.

[0041] One reason why the problem is difficult to solve is that the
deflection of the strip is not known when the floor is being laid or when
it has been taken up and is being laid again, which is one of the major
advantages of the strip lock in comparison with glued joints.
Consequently, it is not possible to solve the problem by adapting in
advance the working measurements of the strip and/or the locking groove
to the curvature of the strip, since the latter is unknown.

[0042] Nor is it preferred to solve this problem by using a wide strip,
whose locking element has a higher locking surface with a larger locking
angle, since a wide strip has the drawback of considerable material
wastage in connection with the forming of the strip. The reason why the
wider but more costly strip works better is mainly because the locking
surface is substantially larger than the maximum strip bending and
because the high locking angle only causes a marginally greater play
which is not visible.

[0043] The strip-bending problems are reinforced by the fact that laminate
flooring is subjected to unilateral moisture influence. The surface layer
and the balancing layer do not co-operate fully, and this always gives
rise to a certain amount of bulging. Concave upward bulging is the
biggest problem, since this causes the joint edges to rise. The result is
an undesirable joint opening between the boards in the upper side of the
boards and high wear of the joint edges. Accordingly, it is desirable to
provide a floorboard which in normal relative humidity is somewhat
upwardly convex by biasing the rear balancing layer. In traditional,
glued floors this biasing is not a problem, rather, it creates a
desirable advantage. However, in a mechanically joined floor with an
integrated strip lock the biasing of the balancing layer results in an
undesirable drawback since the bias reinforces the imbalance of the strip
and, consequently, causes a greater, undesirable backward bending of the
strip. This problem is difficult to solve since the bias is an inherent
quality of the balancing layer, and, consequently, cannot be eliminated
from the balancing layer.

[0044] The invention is also based on a second insight which is related to
the geometry of the joint. We have also discovered that a strip lock with
a relatively deep tongue groove gives rise to greater undesirable bending
of the strip. The reason behind this phenomenon is that the tongue
groove, too, is unbalanced. Consequently, the tongue groove opens when,
in a decrease of the RH, the balancing layer shrinks to a greater extent
than the fibreboard part of the strip, causing the strip to bend
downwards since the strip is an extension of the joint edge below the
tongue groove.

[0045] According to a first aspect of the invention a locking system is
provided of the type which is stated in the first paragraph but one of
the description and which, according to the invention, is characterized
in that the second joint edge, within an area (P) defined by the bottom
of the tongue groove and the locking surface of the locking element, is
modified with respect to the balancing layer.

[0046] Said area P, which is thus defined by the bottom of the tongue
groove and the locking surface of the locking element, is the area which
is sensitive to bending. If the strip bends within this area P, the
position of the locking surface relative to the locking groove, and thus
the properties of the joint, will be affected. Especially, it should be
noted that this entire area P is unbalanced, since nowhere does the part
of the balancing layer located in this area P have a coacting, balancing
surface layer, neither in the tongue groove nor on the projecting strip.
According to the invention, by modifying the balancing layer within this
area P it is possible to change this unbalanced state in a positive
direction, such that the undesirable strip-bending is reduced or
eliminated.

[0047] The term "modified" refers to both (i) a preferred embodiment in
which the balancing layer has been modified "over time", i.e. the
balancing layer has first been applied across the entire area P during
the manufacturing process, but has then been subjected to modifying
treatment, such as milling or grooving and/or chemical working, and (ii)
variants in which the balancing layer at least across part of the area P
has been modified "in space", i.e. that the area P differs from the rest
of the board with respect to the appearance/properties/structure of the
balancing layer.

[0048] The balancing layer can be modified across the entire horizontal
extent of the area P, or within only one or several parts thereof. The
balancing layer can also be modified under the whole of the locking
element or parts thereof. However, it may be preferable to keep the
balancing layer intact under at least part of the locking element to
provide support for the strip against the underlay.

[0049] According to a preferred embodiment, "modifying" means that the
balancing layer is completely or partially removed. In one embodiment,
the whole area P lacks a balancing layer.

[0050] In a second embodiment, there is no balancing layer at all within
one or several parts of the area P. Depending on the type of balancing
layer and the geometry of the joint system, it is, for example, possible
to keep the whole balancing layer or parts thereof under the tongue
groove.

[0051] In a third embodiment, the balancing layer is not removed
completely; it is only reduced in thickness. The latter embodiment can be
combined with the former ones. There are balancing layers where the main
problems can be eliminated by partial removal of some layers only. The
rest of the balancing layer can be retained and helps to increase the
strength and flexibility of the strip. Balancing layers can also be
specially designed with different layers which are adapted in such a way
that they both balance the surface and can act as a support for the strip
when parts of the layers are removed within one area of the rear side of
the strip.

[0052] The modification can also mean a change in the material composition
and/or material properties of the balancing layer.

[0053] Preferably, the modification can be achieved by means of machining
such as milling and/or grinding but it could also be achieved by means of
chemical working, heat treatment or other methods which remove material
or change material properties.

[0054] The invention also provides a manufacturing method for making a
moisture-stable strip-lock system. The method according to the invention
comprises the steps of [0054] forming each floorboard from a body, [0055]
providing the rear side of the body with a balancing layer, [0056]
forming the floorboard with first and second joint edge portions, [0057]
forming said first joint edge portion with [0058] a first joint edge
surface portion extended from the upper side of the floorboard and
defining a joint plane along said first joint edge portion, [0059] a
tongue groove which extends into the body from said joint plane, [0060] a
strip formed from the body and projecting from said joint plane and
supporting at a distance from this joint plane an upwardly projecting
locking element with a locking surface facing said joint plane, [0061]
forming said second joint edge portion with [0062] a second joint edge
surface portion extended from the upper side of the floorboard and
defining a joint plane along said second joint edge portion, [0063] a
tongue projecting from said joint plane for coaction with a tongue groove
of the first joint edge portion of an adjoining floorboard, and [0064] a
locking groove which extends parallel to and at a distance from the joint
plane of said second joint edge portion and which has a downward opening
and is designed to receive the locking element and cooperate with said
locking surface of the locking element.

[0055] The method according to the invention is characterized by the step
of working the balancing layer within an area defined by the bottom of
the tongue groove and the locking surface of the locking element.

[0056] The adaptation or removal of part of the balancing layer in the
joint system can be carried out in connection with the gluing/lamination
of the surface layer, the body, and the balancing layer by displacing the
balancing layer relative to the surface layer. It is also possible to
carry out modifications in connection with the manufacture of the
balancing layer so that the part which will be located adjacent to the
locking system will have properties which are different from those of the
rest of the balancing layer.

[0057] However, a very suitable manufacturing method is machining by means
of milling or grinding. This can be carried out in connection with the
manufacture of the joint system and the floorboards can be
glued/laminated in large batches consisting of 12 or more floorboards.

[0058] The strip-lock system is preferably manufactured using the upper
floor surface as a reference point. The thickness tolerances of the
floorboards result in strips of unequal thickness since there is always a
predetermined measurement from the top side of the strip to the floor.
Such a manufacturing method results in tongue grooves of different depths
in the rear side and a partial removal of a thin balancing layer cannot
be performed in a controlled manner. The removal of the balancing layer
should thus be carried out using the rear side of the floorboard as a
reference surface instead.

[0059] It has also been an object to provide a cost-optimal joint which is
also of high-quality by making the strip as narrow as possible and the
tongue groove as shallow and as strong as possible in order both to
reduce waste since the tongue can be made narrow and to eliminate as far
as possible the situation where the tongue groove opens up and causes
strip-bending as well as rising of the upper joint edge when the relative
humidity changes.

[0060] Known strip-lock systems with a strip of fibreboard and a balancing
layer are characterized in that the shallowest known tongue groove is 3.0
mm in a 7.2-mm-thick floorboard. The depth of the tongue groove is thus
0.42 times the thickness of the floor. This is only known in combination
with a 10.0-mm-wide strip which thus has a width which is 1.39 times the
floor thickness. All other such known strip joints with narrow strips
have a tongue groove depth exceeding 3.6 mm and this contributes
considerably to the strip-bending.

[0061] In order to fulfill the above-mentioned object a strip-lock system
is provided which is characterized in that the tongue groove depth of the
tongue groove and the width of the strip are less than 0.4 and 1.3 times
the floor thickness respectively. This joint affords good joint
properties and especially in combination with high rigidity of the tongue
groove since it can be designed in such a way that as much material as
possible is retained between the upper part of the tongue groove and the
floor surface as well as between the lower part of tongue groove and the
rear side of the floor while, at the same time, it is possible to
eliminate the strip-bending problems as described above. This strip-lock
system can be combined with one or more of the preferred embodiments
which are disclosed in connection with the solution based on a
modification of the balancing layer.

[0062] The opposite joint edge of the board is also unbalanced. In this
case, the problems are not nearly as serious since the surface layer is
not biased and the unbalanced part is more rigid. However, in this case,
too, an improvement can be achieved by making the strip as thin as
possible. This permits minimal removal of material in the locking groove
part of the joint system, which in turn results in maximum rigidity in
this unbalanced part.

[0063] According to the invention there is thus provided a strip-lock
system having a joint geometry characterized in that there is a
predetermined relationship between the width and thickness of the strip
and the height of the locking element on the one hand and the floor
thickness on the other. Furthermore, there is provided a minimum locking
angle for the locking surface. All these parameters separately and in
combination with each other and the above inventions contribute to the
creation of a strip-lock system which can have high joint quality and
which can be manufactured at a low cost.

BRIEF DESCRIPTION OF THE DRAWINGS

[0064] FIGS. 1a-c show in three stages a downward angling method for
mechanical joining of long sides of floorboards according to WO 94/26999.

[0065] FIGS. 2a-c show in three stages a snap-action method for mechanical
joining of short sides of floorboards according to WO 94/26999.

[0066] FIGS. 3a and 3b are a top view and a bottom view respectively of a
floorboard according to WO 94/26999.

[0067] FIG. 4 shows three strip-lock systems available on the market with
an integrated strip of fibreboard and a balancing layer.

[0068] FIG. 5 shows a strip lock with a small tongue groove depth and with
a wide fibreboard strip, which supports a locking element having a large
locking surface and a high locking angle.

[0069] FIG. 6 shows a strip lock with a large tongue groove depth and with
a narrow fibreboard strip, which supports a locking element having a
small locking surface and a low locking angle.

[0070] FIGS. 7 and 8 illustrate strip-bending in a strip lock according to
FIG. 5 and FIG. 6.

[0071] FIG. 9 shows the joint edges of a floorboard according to an
embodiment of the invention.

[0072] FIGS. 10 and 11 show the joining of two floorboards according to
FIG. 9.

[0073] FIGS. 12 and 13 show two alternative embodiments of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0074] Prior to the description of preferred embodiments, with reference
to FIGS. 5-8, a detailed explanation will first be given of the
background to and the impact of strip-bending.

[0075] The cross-sections shown in FIGS. 5 and 6 are hypothetical,
unpublished cross-sections, but they are fairly similar to "Fiboloc®"
in FIG. 4a and "Uniclic" in FIG. 4b. Accordingly, FIGS. 5 and 6 do not
represent the invention. Parts which correspond to those in the previous
Figures are in most cases provided with the same reference numerals. The
design, function, and material composition of the basic components of the
boards in FIGS. 5 and 6 are essentially the same as in embodiments of the
present invention and, consequently, where applicable, the following
description of FIGS. 5 and 6 also applies to the subsequently described
embodiments of the invention.

[0076] In the embodiment shown, the floorboards 1, 1' in FIG. 5 are
rectangular with opposite long sides 4a, 4b and opposite short sides 5a,
5b. FIG. 5 shows a vertical cross-section of a part of a long side 4a of
the board 1, as well as a part of a long side 4b of an adjoining board
1'. The body of the board 1 can be composed of a fibreboard body 30,
which supports a surface layer 32 on its front side and a balancing layer
34 on its rear side. A strip 6 formed from the body and the balancing
layer of the floorboard and supporting a locking element 8 constitutes an
extension of the lower tongue groove part 36 of the floorboard 1. The
strip 6 is formed with a locking element 8, whose operative locking
surface 10 cooperates with a locking groove 14 in the opposite joint edge
4b of the adjoining board 1' for horizontal locking of the boards 1, 1'
transversely of the joint edge (D2). The locking element 8 has a
relatively large height LH and a high locking angle A. The upper part of
the locking element has a guiding part 9 which guides the floorboard to
the correct position in connection with angling. The locking groove 14
has a larger width than the locking element 8, as is evident from the
Figures.

[0077] For the purpose of forming a vertical lock in the direction D1, the
joint edge portion 4a exhibits a laterally open tongue groove 36 and the
opposite joint edge portion 4b exhibits a tongue 38 which projects
laterally from a joint plane F and which in the joined position is
received in the tongue groove 36.

[0078] In the joined position according to FIG. 5, the two adjoining,
upper joint edge surface portions 41 and 42 of the boards 1, 1' define
this vertical joint plane F.

[0079] The strip 6 has a horizontal extent W (=strip width) which can be
divided into: (a) an inner part with a horizontal extent D (locking
distance) which is defined by the joint plane F and a vertical line
through the lower part of the locking surface 10, as well as (b) an outer
part with a horizontal extent L (the width of the locking element). The
tongue groove 36 has a horizontal tongue groove depth G measured from the
joint plane F and inwards towards the board 1 to a vertical limiting
plane which coincides with the bottom of the tongue groove 36. The tongue
groove depth G and the extent D of the locking distance together form a
joint part within an area P consisting of components forming part of the
vertical lock D1 and the horizontal lock D2.

[0080] FIG. 6 shows an embodiment which is different from the embodiment
in FIG. 5 in that the tongue groove depth G is greater, and the strip
width W, the height LH, and the locking angle A of the locking surface
are all smaller. However, the size of the area P is the same in the
embodiments in FIGS. 5 and 6.

[0081] Reference is now made to FIGS. 7 and 8, which show strip-bending in
the embodiments in FIGS. 5 and 6 respectively. The relevant part of the
curvature which may cause problems is the area P, since a curvature in
the area P results in a change of position of the locking surface 10.
Since the area P has the same horizontal extent in both embodiments, all
else being equal, the strip-bending at the locking surface 10 will be of
the same magnitude despite the fact that the strip length W is different.

[0082] The large locking surface 10 and the large locking angle A in FIG.
5 will not cause any major problems in FIG. 7, since the greater part of
the locking surface 10 is still operative. The high locking angle A
contributes only marginally to increased play between the locking element
8 and the locking groove 14. In FIG. 8, however, the large tongue groove
depth G as well as the small locking surface 10 and the low locking angle
A2 create major problems. The strength of the locking system is
considerably reduced and the play between the locking element 8 and the
locking groove 14 increases substantially and causes joint openings in
connection with tensile stress. If the play of the boards is adapted to a
sloping strip at the time of manufacture it may prove impossible to lay
the boards if the strip 6 is flat or bent upwards.

[0083] We have realized that the strip-bending is a result of the fact
that the joint part P is unbalanced and that the shape changes in the
balancing layer 34 and the fibreboard part 30 of the strip are not the
same when the relative humidity changes. In addition, the bias of the
balancing layer 34 contributes to bending the strip 6
backwards/downwards.

[0084] The deciding factors of the strip-bending are the extent of the
locking distance D and the tongue groove depth G. The appearance of the
tongue groove 36 and the strip 6 also has some importance. A great deal
of material in the joint portion P makes the tongue groove and the strip
more rigid and counteracts strip-bending.

[0085] FIGS. 9-11 show how a cost-efficient strip-lock system with a high
quality joint can be designed according to the invention. FIG. 9 shows a
vertical cross-section of the whole board 1 seen from the short side,
with the main portion of the board broken away. FIG. 10 shows two such
boards 1, 1' joined at the long sides 4a, 4b. FIG. 11 shows how the long
sides can be angled together in connection with laying and angled upward
when being taken up. The short sides can be of the same shape.

[0086] In connection with the manufacture of the strip-lock system, the
balancing layer 34 has been milled off both in the entire area G under
the tongue groove 36 and across the entire rear side of the strip 6
across the width W (including the area L under the locking element 8).
The modification according to the invention in the form of removal of the
balancing layer 34 in the whole area P eliminates both the bias and the
strip-bending resulting from moisture movement.

[0087] In order to save on materials, in this embodiment the width W of
the strip 6 has been reduced as much as possible to a value which is less
than 1.3 times the floor thickness.

[0088] The tongue groove depth G of the tongue groove 36 has also been
limited as much as possible both to counteract undesirable strip-bending
and to save on materials. In its lower part, the tongue groove 36 has
been given an oblique part 45 in order to make the tongue groove 36 and
the joint portion P more rigid.

[0089] In order to counteract the effect of the strip-bending and to
comply with the strength requirements, the locking surface has a minimum
inclination of at least 45 degrees and the height of the locking element
exceeds 0.1 times the floor thickness T.

[0090] In order to make the locking-groove part of the joint system as
stable as possible, the thickness SH of the strip in an area
corresponding to at least half the locking distance D has been limited to
a maximum of 0.25 times the floor thickness T. The height LH of the
locking element has been limited to 0.2 times the floor thickness and
this means that the locking groove 14 can be formed by removing a
relatively small amount of material.

[0091] In more basic embodiments of the invention, only the measure
"modification of balancing layer" is used.

[0092] FIG. 12 shows an alternative embodiment for eliminating undesirable
strip-bending. Here, the balancing layer 34 has been completely removed
within the area P (including area G under the tongue groove). However,
under the locking element 8 in the area L the balancing layer is intact
in the form of a remaining area 34', which advantageously constitutes a
support for the locking element 8 against the subfloor. Since the
remaining part 34' of the balancing layer is located outside the locking
surface 10 it only has a marginal, if any, negative impact on the change
of position of the locking surface 10 in connection with strip-bending
and thus changes in moisture content.

[0093] Within the scope of the invention there are a number of alternative
ways of reducing strip-bending. For example, several grooves of different
depths and widths can be formed in the balancing layer within the entire
area P and L. Such grooves could be completely or partially filled with
materials which have properties that are different from those of the
balancing layer 34 of the floorboard and which can contribute to changes
in the properties of the strip 6 with respect to, for example,
flexibility and tensile strength. Filling materials with fairly similar
properties can also be used when the objective is to essentially
eliminate the bias of the balancing layer.

[0094] Complete or partial removal of the balancing layer P in the area P
and refilling with suitable bonding agents, plastic materials, or the
like can be a way of improving the properties of the strip 6.

[0095] FIG. 13 shows an embodiment in which only part of the outer layer
of the balancing layer has been removed across the entire area P. The
remaining, thinner part of the balancing layer is designated 34''. The
part 34' has been left intact under the locking element 8 in the area L.
The advantage of such an embodiment is that it may be possible to
eliminate the major part of the strip-bending while a part (34'') of the
balancing layer is kept as a reinforcing layer for the strip 6. This
embodiment is particularly suitable when the balancing layer 34 is
composed of different layers with different properties. The outer layer
can, for example, be made of melamine and decoration paper while the
inner layer can be made of phenol and Kraft paper. Various plastic
materials can also be used with various types of fibre reinforcement.
Partial removal of layers can, of course, be combined with one or more
grooves of different depths and widths under the entire joint system P+L.
The working from the rear side can also be adapted in order to increase
the flexibility of the strip in connection with angling and snap action.

[0096] Two main principles for reducing or eliminating strip-bending have
now been described namely: (a) modifying the balancing layer within the
entire area P or parts thereof, and (b) modifying the joint geometry
itself with a reduced tongue groove depth and a special design of the
inner part of the tongue groove in combination. These two main principles
are usable separately to reduce the strip-bending problem, but preferably
in combination.

[0097] According to the invention, these two basic principles can also be
combined with further modifications of the joint geometry (c) which are
characterized in that:

[0098] The strip is made narrow preferably less than 1.3 times the floor
thickness;

[0099] The inclination of the locking surface is at least 45 degrees;

[0100] The height of the locking element exceeds 0.1 times the floor
thickness and is less than 0.2 times the floor thickness;

[0101] The strip is designed so that at least half the locking distance
has a thickness which is less than 0.25 times the floor thickness.

[0102] The above embodiments separately and in combination with each other
and the above main principles contribute to the provision of a strip-lock
system which can be manufactured at a low cost and which at the same
affords a high quality joint with respect to laying properties,
disassembly options, strength, joint opening, and stability over time and
in different environments.

[0103] Several variants of the invention are possible. The joint system
can be made in a number of different joint geometry where some or all of
the above parameters are different, particularly when the purpose is to
give precedence to a certain property over the others.

[0104] Applicant has considered and tested a large number of variants in
the light of the above: "smaller" can be changed to "larger",
relationships can be changed, other radii and angles can be chosen, the
joint system on the long side and the short side can be made different,
two types of boards can be made where, for example, one type has a strip
on both opposite sides while the other type has a locking groove on the
corresponding sides, boards can be made with strip locks on one side and
a traditional glued joint on the other, the strip-lock system can be
designed with parameters which are generally intended to facilitate
laying by positioning the floorboards and keeping them together until the
glue hardens, and different materials can be sprayed on the joint system
to provide impregnation against moisture, reinforcement, or
moisture-proofing, etc. In addition, there can be mechanical devices,
changes in the joint geometry and/or chemical additives such as glue
which are aimed at preventing or impeding, for example, a certain type of
laying (angling or snap action), displacement in the direction of the
joint, or a certain way of taking up the floor, for example, upward
angling or pulling along the joint edge.